It is known in the art to prolong the life of spark plug electrodes by attaching precious or noble metal tips to their firing ends. Some of the earliest examples of this technology are seen in U.S. Pat. No. 2,296,033 issued Sep. 15, 1942 to Heller, and in British Patent Specification No. 479,540 published in 1938 to Powell et al. The Heller patent teaches the attachment of precious metal tips to ground and center electrodes formed of much less expensive materials. The precious metal tips are comprised of corrosion resistant materials, including platinum alloys such as platinum-rhodium, platinum-iridium and platinum-ruthenium. Similarly, the Powell reference discloses the use of platinum, iridium, ruthenium, osmium and alloys thereof, including iridium-rhodium, for use as firing tips for spark plug electrodes. In the time since this and other early designs, there have sprung numerous other inventions attempting to utilize the corrosion and erosion resistant properties of noble and other precious metals.
For many years, platinum was the precious metal of choice for spark plug electrode firing tips, as evidenced by the numerous patents describing its use. During recent years, however, numerous other noble metals and noble metal alloys have become more frequently utilized; one of which is iridium. Iridium can be relatively inexpensive, when compared to other comparable noble metals, and has the rather high melting point of approximately 2410° C. Though many benefits exist regarding the use of iridium, it is sometimes a challenge to work with during manufacturing, as it has a tendency to crack under mechanical pressure and deformation. In order to overcome this and other challenges, various iridium-alloys have been developed with the hope of imparting certain, desirable characteristics to the metal. An example of such an alloy is taught in U.S. Pat. No. 6,094,000 issued Jul. 25, 2000 to Osamura et al. In this reference there is disclosed an Ir—Rh alloy whose relative percentages of iridium and rhodium vary according to one of several embodiments.
In addition to the electrode tip composition, removing heat from the noble metal tips of the center and/or ground electrode has proven to be an effective approach to increasing the operational life of those tips. The combustion process produces a great amount of intense heat to which the noble metal tips are exposed. Experience has shown that intense heat, for example in excess of 1000° C., can increase oxidation, chemical corrosion or electrical erosion, which can contribute to the accelerated deterioration of the noble metal tips. It is desirable to construct the firing end components, such as the noble metal tip, center or ground electrode, insulator or shell, according to certain designs and with a certain combination of dimensions such that it reduces the aforementioned accelerated deterioration, as well as other unwanted phenomenon such as carbon fouling, etc. U.S. Pat. No. 6,147,441 and U.S. Application Publication 2003/0071552 A1 are examples of references that disclose a spark plug design utilizing noble metal inserts and a series of preferred dimensions.
Thus, it would be advantageous to provide a spark plug having an improved noble metal tip located at its firing end, preferably an iridium or iridium-alloy tip, wherein the spark plug further includes firing end components designed to extend the operational life and other performance characteristics of the spark plug.